clarified comments
updated pom.xml: adding joda-time as a dependency

using joda-time (LocalTime, LocalDate, Period) for activity datetime
info; much simpler than the mess that is util.Calendar and it's ilk
Activity classes now use joda-time

(also, model -> domain package rename)
TDD: initial basic tests
constructor(s), getters, setters for (Single)Activity
toString, Activity -> AbstractActivity rename
Elaborated Test Case for toString()
...more progress
basic tests for WeeklyActivity
initial skeleton for WeeklyActivity
toString for WeeklyActivity etc
first exception test! yay!"
progress diary updated
More testing for constructor&setter arguments
TDD: check beginDate doesn't end up after endDate

This should never happen -> IllegalArgumentException
Ensuring beginTime doesn't end up after endTime

however, this might be subject to change (see comments)

making tests pass, too
Rename AbstractActivity -> Activity

Previous naming redundant. However, consider returning to it if
a need for an interface "ActivityInterface" arises...?
initial skeleton of CourseModule class
Merge branch 'flesh-out-domain-objects'

* the most fundamental classes (Activity, WeeklyActivity,
  SingleActivity) are go
* they come with sets with basic tests